1. Field of the Invention
The present invention relates to control apparatus and method for feeding a melt to a coinjection hot runner system. Preferably, the present invention utilizes a Dynamic Clamp Feed (DCF) to operate at least one of the coinjection shooting pots for injecting at least two melt materials into a mold cavity. Preferably, one of the melt materials is caused to reverse flow into the nozzle melt channel of one of the other materials during the process. Preferably, this decompression step occurs after the refilling of the shooting pots.
2. Description of Related Art
Coinjection molding is typically used to mold multilayered plastic packaging articles having a laminated wall structure. Each layer is typically passed through a different annular or circular passageway in a single nozzle structure and each layer is partially, sequentially, injected through the same gate. Some coinjection hot runner systems include shooting pots to meter material of one plastic resin so that each cavity of a multi-cavity mold receives an accurate dose of that resin in the molding cycle. Such systems may also use shooting pots to exert supplementary pressure on the melt during the molding process. In such systems, a check valve is often used to prevent backflow of resin into the shooting pot during the injection of the resin into the mold cavity.
U.S. Pat. Nos. 4,609,516 and 4,990,301, both to Krishnakumar, disclose coinjection molding processes employing hot runner systems that use shooting pots. Both of these patents disclose sequence charts that show the sequence in which the multiple materials are injected into the mold cavity.
U.S. Pat. No. 6,152,721 to Schad discloses a shooting pot actuator mechanism for operating the shooting pots of a coinjection hot runner system. See also:
U.S. patent application Ser. No. 10/879,576 entitled INJECTION MOLDING MACHINE SHOOTING POT WITH INTEGRAL CHECK VALVE;
U.S. patent application Ser. No. 10/879,581 entitled INJECTION MOLDING MACHINE SPIGOTTED SHOOTING POT PISTON;
U.S. patent application Ser. No. 10/879,621 entitled APPARATUS AND METHOD FOR SEALING INJECTION UNIT AND SPRUE;
U.S. patent application Ser. No. 10/879,575 entitled APPARATUS AND METHOD FOR ACTUATION OF INJECTION MOLDING SHOOTING POTS;
U.S. patent application Ser. No. 10/880,494 entitled HOT RUNNER COINJECTION NOZZLE WITH THERMALLY SEPARATED MELT CHANNELS;
U.S. patent application Ser. No. 10/880,493 entitled COINJECTION MOLDING COOLED SHOOTING POT CYLINDER; and
U.S. patent application Ser. No. 10/887,353 entitled APPARATUS AND METHOD FOR INJECTION MOLDING SHOOTING POT WEDGE FEATURE.
With current coinjection nozzle/shooting pot configurations, however, resin material that is trapped between the check valve and the valve gate during the different molding cycle steps remains under pressure. This often results in unwanted drooling or leakage of resin into the valve gate and/or the mold cavity. In more detail, if a second resin in a coinjection nozzle melt channel remains under pressure when the valve gate stem is withdrawn to inject the next shot of the first resin, a portion of the second resin moves to the front of its melt channel and maybe into the gate area. Then, when the next shot of the first resin moves through the gate, it drags along that portion of the second resin. The presence of the second resin in the shot of the first resin may result in a defective part. No known art discloses any means for relieving this build up of pressure between the check valve and the valve gate. Adapting new structures to act as a pressure relief valve would add complicated mechanical structure to the molding machine, leading to increased manufacturing and maintenance costs.
Thus, what is needed is a coinjection molding control structure which can relieve the pressure build up between the check valve and the valve gate during the molding cycle. Preferably, such a solution will require a minimum of new hardware and/or software to implement and maintain.